Little, a former competitive boxer, would knock his victims out with punches before strangling them - meaning that there were not always "obvious signs" that the person had been killed.

Now, they are hoping that Little's drawings can help them to finally find out who the victims were so that their families can be notified.

"With no stab marks or bullet wounds, many of these deaths were not classified as homicides but attributed to drug overdoses, accidents, or natural causes," the FBI said in its initial report in November last year.

Image copyrightFBIImage caption
'White female between 20-25 years old killed in 1972. Victim possibly from Massachusetts'

Little's case was passed on to the FBI's Violent Criminal Apprehension Programme (ViCAP), which analyses people who serially commit violent and and sexual crimes. They then share their findings with local law enforcement in different areas, in order to check them against any unsolved crimes.

ViCAP, tasked with doing a full background check on Little, noticed that the three LA killings were very similar to a number of unsolved deaths dating back to the 1970s.

Image copyrightFBIImage caption
'Black female between 35-45 years old killed in 1977. Met the victim in Gulfport, Mississippi. Victim possibly from Pascagoula. Victim possibly worked at Ingalls Shipyard.'

Crime analyst Christina Palazzolo writes on the FBI website that they "found a case out of Odessa, Texas, that sounded very much like him, and we could place him passing through the area around the same time".

In spring last year, investigators set up an interview with Little, hoping to find out more information. Knowing that he wanted to move prisons, they struck a deal - he could move prisons if he talked.

Then, during the interview, Ms Palazzolo says "he went through city and state and gave [us] the number of people he killed in each place". Once he was done, he had confessed to 90 killings. The FBI says it has so far been able to verify 34 of these.

Many of Little's victims were sex workers, people with substance abuse issues and trans women, whose deaths may not have been investigated or would have been ruled to be accidental at the time.

His memory of the killings was mostly precise, as he could give details about where they happened and what car he was driving. But he was unable to remember specific dates - which, investigators say, has caused further issues with identifying the victims.

Agents are continuing to question Little and collect drawings of his victims.

Other images are described as:

Las Vegas, Nevada: 'Black female, age 40, killed in 1993'

Monroe, Louisiana: 'Black female, age 24, killed between 1987 and the early 1990s'